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Discussion Starter #1 (Edited)
Here some info for those of you having this problem.
There could be many reasons for it not to start as you have already read throughout the endless Forums.

Here was my ordeal..
a friend brought over his 2007 trx450er after two local shops could not get it to run.
He said it ran fine last time it ran, then no start on the next outing.

The first shop said it was electrical. so the tore through it and still could not get it to run. they told him to come get it.

The second shop said it had low compression among other issues.
they un wrapped the wiring because they also said it must have a short somewhere.
after finding nothing they said low compression is the cause. they told him they put a new piston in and it still would not start and it was still back firing.

They said they checked the timing and valve clearance.
It sat in the shop for 6 month with no resolution.
So he picked it up from the second shop in boxes because the tore it all back down because they were at a loss.

After going through everything, i found there were parts missing and
he had to make 3 trips to back to get the lost parts from a complete bike they had.

Now my turn..
first thing i did was go through the un wrapped wiring and tested all the plug ends, then re wrapped everything.

Then i looked at the piston they said they put in. NOPE no new piston. they cleaned the old one up.
there is no way they tried it with a new piston and why would they take it out ?

So i ordered a new piston and cylinder kit.

I did a valve job on the head and re assembled the motor.
with everything back together i tried to start it.

Nope, same thing was happening he said was happening before.. it will turn over and pop out the exhaust and intake.
So i pulled it all apart and the valves and timing are spot on.

Tried again and the same is happening.
I bypassed the tcp and crossed the wires on the carb. still no start.
Now i am wondering what they did to it at the shops.

I went through this quad two more times. i even tried a different carb to see if it was a fuel problem.
Nope.. still backfiring.


After letting it sit for a week, i jumped back into it wondering what it could be.
I put in a new cdi , driftwood key and stater. the wires by the stater and cdi plugs had been stripped for testing by the other shops.
"could they have the shorted out by bare wires touching ? who knows "

I also put in a new starter because it was burnt up from the other shops trying to start it.

All back together and Still no start. ?? WTF.. it has spark, it has fuel, timing is correct, valves are exact, it has compression and i set the decomp to 55.
I was beating my self in the head. there is no reason this should not start.

a few days later i was back on it trying to get it to run.
then i remembered something that happened to my on one of my bikes years ago.

My two stroke banshee stopped running on one cylinder.
it had the same symptoms.

So i replaced the spark plug boot on this quad and it started right up and runs fine now.

You would not think that the boot could go bad, and still put out a spark.
what happens is the bad connection in the boot will delay the spark long enough to fire after it should, making it back fire and seem the timing is off.

So if your bike was running fine last time you road it and now it wont start and has this symptom,
try changing the plug boot from the coil first.

Moisture over time will cause this Boot to short out inside.

hope this works for someone..
 

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The simple problems are the ones most overlooked, but this post contains some mis-information due to common mis-conceptions.

The spark plug cap has a 5K ohm resistor built into it. The purpose of the resistor is to delay the spark by microseconds, but the delay provides a stronger spark at the plugs electrodes. The resistor can fail for various reasons including heat, vibration, improper removal from the spark plug and moisture. The reason it is overlooked is when the spark plug is outside the cylinder it will spark, but inside the cylinder where the air/fuel mixture is under compression and turbulence, the spark potential may be too weak to ionize the air between the electrodes to allow the spark to jump the gap.

Both shops failed to follow basic diagnostic procedures and the second shop failed miserably - I would not trust the second shop to change the oil properly!

The problem may have been quickly diagnosed with a timing light - if the spark does not jump the gap at the plug, the light will not flash and the loss of spark will be evident.

Congratulations on finding the problem, but you also failed the basic troubleshooting diagnostics. You resolving the problem was more luck than knowledge, but the result was the same.

And for the rest of novice mechanics, take note.
 

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Discussion Starter #3
I agree with you. i am not a full time motorcycle mechanic. i don't have all the tools to trouble shoot every problem. I didn't have a shop manual to go by either.
i was going by what i know of the basics. doing research on the internet i never ran across any post saying there was a resister in the cap. i thought because it was sparking out side the motor, it was inside also. the spark seemed to be a good and strong outside. anyhow. we did not have to pay a shop mega $$$ to fix this. i did not like to look of the cylinder with the wear lip on the top and who knows what they did to the piston cleaning it up. i guess the good thing for him is that everything is new again and should last him a long time. i had asked him before hand how far he wasted to go with this fixing it. he said he paid $1200 for it from his brother in law just before it stopped running. we put about $800 in to all new parts. so he's in it for $2K and could still sell it and make a little profit off it. .
 

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$800 total and a rebuilt engine is not bad, but yeah it was a lot of time wasted - still good that you found the problem as it was overlooked by the pros - the only difference between a pro and a novice is the pro gets paid to do something and the novice gets paid the satisfaction of the outcome.
 

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Discussion Starter #5
$800 total and a rebuilt engine is not bad, but yeah it was a lot of time wasted - still good that you found the problem as it was overlooked by the pros - the only difference between a pro and a novice is the pro gets paid to do something and the novice gets paid the satisfaction of the outcome.
Yes i know what you mean. He was happy but i felt a total weight lifted off my shoulders..
 
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